The present invention is in the nonlinear optical field, and particularly concerns nonlinear processes to extend or alter the frequency regime of laser radiation, monitor electric fields, and provide electric-based motion control. The second harmonic response of silicon nanoparticle microcystals also provides a basis for sensors in biological applications, namely, second harmonic imaging applications.
Controlled lasers are the basis for a wide variety of modern devices useful in fields ranging from medical devices, such as surgical lasers, to communication devices that use light as a data carrier instead of electricity. Reliance on optical energy, and particularly laser radiation, continues to increase. Optical systems, compared to electrical systems, have higher data carrying capacity, do not suffer from electromagnetic interference, are faster, and have many other benefits known to those skilled in the art.
Semiconductor lasers produce laser light from a semiconductor structure. Such devices emit laser lights at a frequency inherent to the material used, typically, Group III-V semiconductors. Altering, and particularly increasing, the frequency expands device usefulness. Extra frequencies offer increased communication or bands, for example, and higher frequency lasers perform work more efficiently.
In general, the nonlinear polarization for a material can be expressed as P="khgr"(1)E1+"khgr"(2)E2+"khgr"(3)E3 . . . where P is the induced polarization, "khgr"(n) is the nth order nonlinear susceptibility, and E is the electric field vector. The first term describes normal absorption and reflection of light; the second term describes the second harmonic generation (SHG), sum and difference frequency generation; and the third, describes light scattering, stimulated Raman processes, third harmonic generation (THG), both two- and three-photon absorption. SHG does not arise from an absorptive process. Instead, an intense laser field induces a nonlinear polarization in a molecule or assembly of molecules, resulting in the production of a coherent wave at exactly twice the incident frequency (half the wavelength). The magnitude of the SHG wave can be resonance enhanced when the energy of the photon energy of the second harmonic wave overlaps with an electronic absorption band.
A major constraint of SHG is the requirement of a noncentrosymmetric environment. This is readily understood as follows. The second harmonic wave is a vector quantity, and within the electric dipole approximation, the induced polarization in a centrosymmetric sample from one direction would be equal and opposite the other, thus canceling. Efficient crystals used today in harmonic generation include KH2PO4 (KDP), NH4H2PO4 (ADP), LiNbO3, Ba2NaNb5O15. The crystals are hydrophilic, requiring encapsulation in phase matched materials, and they are plagued by incident laser and temperature damage.
There has been recent interest in the nonlinear optical response in silicon nano material. The activity in silicon was stimulated by the discovery of the optical activity of porous silicon and the associated nanoscale structure of the material. In measurements by Wang et. al. using 50 picosecond pulsed 1.06 xcexcm excitation, it was suggested that a third harmonic photon (at 355 nm) is first generated in the core of the nanostructures. The photoluminescence is then produced by a single photon excitation by the internally generated UV radiation. However, no radiation at the third harmonic was directly detectable in these experiments. More recent measurements by Chin et al. using short pulse at 0.870, 1.06 and 1.3 xcexcm radiation there was no evidence for second or third harmonic generation. The internal generation of second harmonic photon was ruled out since second harmonic generation is not allowed in bulk silicon due to the centrosymmetry. On the theoretical side, silicon is known to have negligible nonlinearity, being zero at the second order level (not allowed), and very small at the third order level. However, calculations show that the nonlinear polarizability of semiconductors may be greatly enhanced, by several order of magnitude, over bulk values when the crystallite size is reduced to the nano meter regime. However, for systems of large dielectric constants and small effective masses such as silicon, it was ruled out. It was argued excitonic Bohr radius is as large as 4 nm, and the quantization energies of the unbound electron and hole become, in ultrasmall nanoparticles (1-3 nm across), easily larger than the exciton binding energy so that the conditions for enhancement are not satisfied.
Thus, there is a need for improved nonlinear optical devices. There is a further need for a method for harmonic generation, and an improved crystal structure for harmonic generation.
The present invention is directed to these needs, and meets the needs by demonstrating nonlinear optical devices and harmonic generation based upon silicon nanoparticle microcrystal structures. The nonlinear response of silicon microcrystals provides for frequency doubling at incident wavelengths from about 600 nm to 1000 nm and frequency mixing is also provided at the same range of incident wavelengths.
We report the first observation of second harmonic generation in microcrystals of ultrasmall silicon nanoparticles of the invention. The results are surprising since harmonic generation is not allowed in bulk silicon due to the centrosymmetry and theory predicts similar behavior for other forms of silicon.
The present invention relies upon a previously unknown material, silicon nanoparticles. This new material and a method for making the same are described in copending application serial number Ser. No. 09/426,389, to Nayfeh et al. entitled SILICON NANOPARTICLES AND METHOD OF MAKING THE SAME, which is incorporated by reference herein. Electrochemical etched silicon is dispersed into a suspension nano particle colloid of xcx9c1 nm across. The particles of the invention were then reconstituted to create large, thick, uniform film layers of faceted micro-crystallites. This type of microcrystals and films thereof emit the second harmonic of incident radiation, which has also been observed from individual microcrystals of the invention. The silicon nanoparticle microcrystal is accordingly the basis for many nonlinear optical devices.
Particular devices of the invention include frequency doublers and frequency mixers. Doublers and mixers may be incorporated in many optoelectronic applications. In addition, the frequency response of silicon microcrystals of the invention provides a basis for an electrochromatic semiconductor device usable, for example, to measure electric fields. The microcrystals also exhibit piezoelectric characteristics, providing a new piezoelectric material suitable, for example, for electric based motion control. Because nonlinear emission is coherently related to the incident, collimated, and of width that is basically limited by the bandwidth of the incident radiation, it can provide superior detection capabilities as a biosensing marker, improving the signal to background tremendously in second harmonic imaging (SHIM).